Conventional image forming apparatuses include an example in which remaining amounts of toner in toner containers are detected using piezoelectric sensors or ultrasonic sensors. For example, a remaining toner amount detection apparatus described in Japanese Patent Application Laid-Open No. H1-6986 includes a piezoelectric sensor installed, with a detection unit of the sensor facing upward, at that position on a bottom face of a hopper near which a thin plate member passes during rotation of an agitation member and detects the remaining amount of toner based on the time during which pressure is detected by the sensor to the time required for one rotation of the agitation member. With the remaining toner amount detection apparatus, when the remaining amount of toner is equal to or larger than a certain amount, output of the piezoelectric sensor is fixed to a Toner Present logic. On the other hand, when the remaining amount of toner is equal to or smaller than a certain amount, the amount of toner cannot be detected and the output of the piezoelectric sensor is fixed to a No Toner logic. However, the detection method disclosed in Japanese Patent Application Laid-Open No. H1-6986 has the following problems. Specifically, when there is a large amount of remaining toner, there is no period of time during which toner weight is not detected, and thus the remaining amount of toner cannot be detected until the toner decreases to a predetermined amount. Also, along with recent increases in the speed of image forming apparatus, when an agitation member operates at high speed, the toner in a toner container whirls up, causing toner to exist at a detection position of the piezoelectric sensor and thereby making it difficult to secure a period of time during which toner weight is not detected.
Conventional image forming apparatuses include one that uses a magnetic permeability sensor to detect the amount of toner in a developing unit. Examples of the apparatus which detects the amount of toner using a magnetic permeability sensor include one described in Japanese Patent Application Laid-Open No. 2002-132036. Japanese Patent Application Laid-Open No. 2002-132036 discloses a toner amount detection apparatus which includes a first agitation blade configured to be flexible and adapted to deform in a direction opposite to a rotational direction when agitating toner, a second agitation blade configured to be rigid and placed behind the first agitation blade in the rotational direction, and a magnetic permeability sensor placed on an outer bottom face of a developing unit. The apparatus detects states of rotating operations of the metal materials placed on the respective agitation blades using the magnetic permeability sensor placed on the outer bottom face of the developing unit. Also, the apparatus is configured such that the first agitation blade and second agitation blade rotate integrally when there is a large amount of toner in the developing unit and that the first agitation blade and second agitation blade rotate separately without deformation when there is a small amount of toner in the developing unit. In so doing, variation in magnetic permeability per rotation of a rotating shaft is measured once using the magnetic permeability sensor when there is a large amount of toner in the developing unit, and twice when there is a small amount of toner in the developing unit. The toner amount detection apparatus detects the amount of toner in the developing unit based on variation in the number of detections.
However, the apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-132036 has the following problem. When there is a large amount of toner, since the first agitation blade and second agitation blade rotate integrally, a signal detected by the magnetic permeability sensor represents one variation in magnetic permeability per rotation of the rotating shaft. On the other hand, when there is a small amount of toner, the first agitation blade hardly deforms and the first and second agitation blades do not rotate integrally. In this case, a signal detected by the magnetic permeability sensor represents two variations in magnetic permeability per rotation of the rotating shaft. In this way, based on the number of magnetic field variations detected by the magnetic permeability sensor (once or twice), the amount of toner is detected alternatively by selecting between a large amount and small amount or between presence and absence. Thus, it is difficult to successively detect the variation in the amount of toner.